Specific emitter identification (SEI) is an important part of radio frequency (RF) electronic warfare (EW). The objective of SEI is to discern between and uniquely identify multiple specific RF emitters even when the emitters transmit the same type of signal. This can be achieved by estimating a set of RF features that arise from variations or imperfections in the electronic components of an emitter used to synthesize and transmit a signal. This set of estimated features is then used to identify specific emitters.
SEI is greatly complicated when the emitters of interest are part of a code division multiple access (CDMA) system. This is because there may be no separation between emitters in either time or frequency. Spatial separation may be of some use, but cannot be relied upon in all situations.
In addition, SEI can be greatly complicated when the emitters of interest are analog frequency modulation (FM) push-to-talk (PTT) voice radios. This is because the modulating signal (human voice) is non-stationary and has no simple structure that can be exploited.